Oxiranyl-acyl derivatives as additives for electrolytes in lithium ion batteries

ABSTRACT

The present invention relates to an electrolyte composition (A) containing (i) at least one aprotic organic solvent; (ii) at least one conducting salt; (iii) at least one compound of formula (I) and (iv) optionally at least one further additive.

The present invention relates to an electrolyte composition (A)containing

-   (i) at least one aprotic organic solvent;-   (ii) at least one conducting salt;-   (iii) at least one compound of formula (I)

-   -   wherein    -   X, R¹, R², R³, R⁴, and n are defined below, and

-   (iv) optionally at least one further additive.

The present invention further relates to the use of compounds of formula(I) as additives for electrolytes in electrochemical cells and toelectrochemical cells comprising the above described electrolytecomposition (A), at least one cathode (B) comprising at least onecathode active material, and at least one anode (C) comprising at leastone anode active material.

Storing electrical energy is a subject of still growing interest.Efficient storage of electric energy would allow electric energy to begenerated when it is advantageous and used when needed.

Accumulators, for example lead accumulators and nickel-cadmiumaccumulators, have been known for many decades. The known leadaccumulators and nickel-cadmium accumulators have the disadvantages,however, of a comparatively low energy density and of a memory effectwhich reduces the rechargeability and hence the useful life of leadaccumulators and nickel-cadmium accumulators.

Lithium ion accumulators, frequently also referred to as lithium ionbatteries, are used as an alternative. They provide higher energydensities than accumulators based on lead or comparatively noble heavymetals.

Since many lithium ion batteries utilize metallic lithium or lithium inoxidation state 0, or produce it as an intermediate, they are watersensitive. Moreover, the conductive salts used, for example LiFP₆, arewater sensitive during long-term operation. Water is therefore not ausable solvent for the lithium salts used in lithium ion batteries.Instead, organic carbonates, ethers, esters and ionic liquids are usedas sufficiently polar solvents. Most state of the art lithium ionbatteries in general comprise not a single solvent but a solvent mixtureof different organic aprotic solvents.

During charge and discharge of lithium ion batteries various reactionstake place at different cell potentials. It is known that during thefirst charging process of a lithium ion battery usually a film is formedon the anode. This film is often called solid electrolyte interface(SEI). The SEI is permeable for lithium ions and protects theelectrolyte from direct contact with the anode and vice versa. It isformed by reductive decomposition of components of the electrolytecomposition like solvents, e.g. carbonates, esters, and ethers, andconductive salts on the surface of the anode, especially if the anodeactive material is a carbonaceous material like graphite. A certainamount of the lithium present in the cell is irreversibly consumed forthe formation of the SEI and cannot be replaced. One possibility toreduce the amount of irreversibly consumed lithium is the addition ofsuitable chemical compounds which are easily decomposed on the anode byreduction and thereby forming a film on the surface of the anode. Oneespecially well suited compound is vinylene carbonate, see for instanceEP 0 683 587 B1 and U.S. Pat. No. 6,413,678 B1. Vinylene carbonate formsa stable SEI on a graphite anode in lithium ion batteries.

Other film forming additives are known, inter alia oxiranyl derivativesas described in US 2009/0035656 A1 disclosing glycidyl ether compoundsas film forming additives for electrolytes of lithium batteries.

Nevertheless there is still the need for enhancing the lifetime ofsecondary batteries and a demand for electrolyte additives leading to aprolonged life time and cycle stability of secondary electrochemicalcells.

It was an object of the present invention to provide an electrolytecomposition leading to an improved lifetime of electrochemical cells, inparticular lithium ion batteries. A further object of the presentinvention was to provide electrochemical cells, in particular lithiumion batteries of high energy density and/or higher operating voltagehaving good performance characteristics and long lifetime.

This object is achieved by an electrolyte composition (A) containing

-   (i) at least one aprotic organic solvent;-   (ii) at least one conducting salt;-   (iii) at least one compound of formula (I)

wherein

-   -   X is either carbon, S(O) or P(OR⁸);    -   n can be 0 or 1;    -   R¹ is selected from C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆        alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇        (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl,        (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl,        alkynyl, and (hetero)aryl may be substituted by one or more        substituents selected from F, CN, C₁-C₆ alkyl, C₃-C₆        (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁵,        C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵, OS(O)₂R⁵, and        S(O)₂OR⁵;    -   R², R³ and R⁴ are independently from each other selected from H,        C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆        (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and        C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl,        (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl,        alkynyl, and (hetero)aryl may be substituted by one or more        substituents selected from F, CN, C₁-C₆ alkyl, C₃-C₆        (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷,        C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and        S(O)₂OR⁷; or R² and R³ are combined with each other and form        together with the C—C single bond of the oxirane cycle a 5 to 7        membered hydrocarbon cycle which is substituted by at least one        group selected from (O) and oxiranyl, and/or two adjacent        C-atoms of the hydrocarbon carbon cycle form together with an        O-atom an additional oxirane cycle, and R⁴ is defined as above;    -   R⁵, R⁶, R⁷ and R⁸ are independently from each other selected        from H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl,        C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl,        C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl,        alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl may be        substituted by one or more substituents selected from F, CN, and        oxiranyl; and

-   (iv) optionally at least one further additive.

The problem is further solved by the use of at least one compound offormula (I) as additive for electrolytes in electrochemical cells; andby the electrochemical cell comprising the electrolyte composition (A)as described above, at least one cathode (B) comprising at least onecathode active material, and at least one anode (C) comprising at leastone anode active material.

The compounds of general formula (I) show high reduction potentialsindicating their suitability as additives forming a film on the anode ofsecondary electrochemical cells. Electrolytes for lithium ion secondarybatteries comprising at least one aprotic organic solvent or a mixturethereof, at least one conducting salt leads and at least one compound ofgeneral formula (I) show good values of the internal resistance and verysmall decrease of capacity retention during cycling.

The inventive electrolyte composition (A) is preferably liquid atworking conditions; more preferred it is liquid at 1 bar and 25° C.,even more preferred the electrolyte composition is liquid at 1 bar and−15° C., in particular the electrolyte composition is liquid at 1 barand −30° C., even more preferred the electrolyte composition is liquidat 1 bar and −50° C.

The electrolyte composition (A) contains at least one aprotic organicsolvent (i), preferably at least two aprotic organic solvents (i).According to one embodiment the electrolyte composition (A) may containup to ten aprotic organic solvents (i).

The at least one aprotic organic solvent (i) is preferably selected from

-   (a) cyclic and acyclic organic carbonates, which may be partly    halogenated,-   (b) di-C₁-C₁₀-alkylethers, which may be partly halogenated,-   (c) di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers and polyethers, which may    be partly halogenated,-   (d) cyclic ethers, which may be partly halogenated,-   (e) cyclic and acyclic acetals and ketals, which may be partly    halogenated,-   (f) ortho esters, which may be partly halogenated,-   (g) cyclic and acyclic esters of carboxylic acids, which may be    partly halogenated,-   (h) cyclic and acyclic sulfones, which may be partly halogenated,-   (i) cyclic and acyclic nitriles and dinitriles, which may be partly    halogenated, and-   (j) ionic liquids, which may be partly halogenated.

More preferred, the at least one aprotic organic solvent (i) is selectedfrom cyclic and acyclic organic carbonates (a), di-C₁-C₁₀-alkylethers(b), di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers and polyethers (c) and cyclicund acyclic acetals and ketals (e), even more preferred electrolytecomposition (A) contains at least one aprotic organic solvent (i)selected from cyclic and acyclic organic carbonates (a) and mostpreferred electrolyte composition (A) contains at least two aproticorganic solvents (i) selected from cyclic and acyclic organic carbonates(a), in particular preferred electrolyte composition (A) contains atleast one aprotic solvent (i) selected from cyclic organic carbonatesand at least one aprotic organic solvent (i) selected from acyclicorganic carbonates.

The aprotic organic solvents (a) to (j) may be partly halogenated, e.g.they may be partly fluorinated, partly chlorinated or partly brominated,preferably they may be partly fluorinated. “Partly halogenated” means,that one or more H of the respective molecule is substituted by ahalogen atom, e.g. by F, Cl or Br. Preference is given to thesubstitution by F. The at least one solvent (i) may be selected frompartly halogenated and non-halogenated aprotic organic solvents (a) to(j), i.e. the electrolyte composition may contain a mixture of partlyhalogenated and non-halogenated aprotic organic solvents.

Examples of suitable organic carbonates (a) are cyclic organiccarbonates according to the general formula (a1), (a2) or (a3)

wherein

-   R^(a), R^(b) und R^(c) being different or equal and being    independently from each other selected from hydrogen; C₁-C₄-alkyl,    preferably methyl; F; and C₁-C₄-alkyl substituted by one or more F,    e.g. CF₃.

“C₁-C₄-alkyl” is intended to include methyl, ethyl, n-propyl,iso-propyl, n-butyl, iso-butyl, sec.-butyl and tert.-butyl.

Preferred cyclic organic carbonates (a) are of general formula (a1),(a2) or (a3) wherein R^(a), R^(b) and R^(c) are H. Examples are ethylenecarbonate, vinylene carbonate, and propylene carbonate. A preferredcyclic organic carbonate (a) is ethylene carbonate. Further preferredcyclic organic carbonates (a) are difluoroethylene carbonate (a4) andmonofluoroethylene carbonate (a5)

Examples of suitable acyclic organic carbonates (a) are dimethylcarbonate, diethyl carbonate, methylethyl carbonate and mixturesthereof.

In one embodiment of the invention the electrolyte composition (A)contains mixtures of acyclic organic carbonates (a) and cyclic organiccarbonates (a) at a ratio by weight of from 1:10 to 10:1, preferred offrom 3:1 to 1:1.

Examples of suitable acyclic di-C₁-C₁₀-alkylethers (b) aredimethylether, ethylmethylether, diethylether, diisopropylether, anddi-n-butylether.

Examples of di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers (c) are1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme (diethylene glycoldimethyl ether), triglyme (triethylene glycol dimethyl ether),tetraglyme (tetraethylene glycol dimethyl ether), and diethylene glycoldiethyl ether.

Examples of suitable polyethers (c) are polyalkylene glycols, preferablypoly-C₁-C₄-alkylene glycols and especially polyethylene glycols.Polyethylene glycols may comprise up to 20 mol % of one or moreC₁-C₄-alkylene glycols in copolymerized form. Polyalkylene glycols arepreferably dimethyl- or diethyl-end-capped polyalkylene glycols. Themolecular weight M_(w) of suitable polyalkylene glycols and especiallyof suitable polyethylene glycols may be at least 400 g/mol. Themolecular weight M_(w) of suitable polyalkylene glycols and especiallyof suitable polyethylene glycols may be up to 5 000 000 g/mol,preferably up to 2 000 000 g/mol.

Examples of suitable cyclic ethers (d) are tetrahydrofurane and1,4-dioxane.

Examples of suitable acyclic acetals (e) are 1,1-dimethoxymethane and1,1-diethoxymethane.

Examples for suitable cyclic acetals (e) are 1,3-dioxane and1,3-dioxolane.

Examples of suitable ortho esters (f) are tri-C₁-C₄ alkoxy methane, inparticular trimethoxymethane and triethoxymethane. Examples of suitablecyclic ortho esters (f) are1,4-dimethyl-3,5,8-trioxabicyclo[2.2.2]octane and4-ethyl-1-methyl-3,5,8-trioxabicyclo[2.2.2]octane.

Examples of suitable acyclic esters of carboxylic acids (g) are ethylacetate, methyl butanoate, and esters of dicarboxylic acids like1,3-dimethyl propanedioate. An example of a suitable cyclic ester ofcarboxylic acids (lactones) is γ-butyrolactone.

Examples of suitable cyclic and acyclic sulfones (h) are ethyl methylsulfones, dimethyl sulfone and tetrahydrothiophene-S,S-dioxide.

Examples of suitable cyclic and acyclic nitriles and dinitriles (i) areadipodinitrile, acetonitrile, propionitrile, butyronitrile.

The water content of the inventive electrolyte composition is preferablybelow 100 ppm, based on the weight of the electrolyte composition, morepreferred below 50 ppm, most preferred below 30 ppm. The water contentmay be determined by titration according to Karl Fischer, e.g. describedin detail in DIN 51777 or ISO760: 1978.

The content of HF of the inventive electrolyte composition is preferablybelow 60 ppm, based on the weight of the electrolyte composition, morepreferred below 40 ppm, most preferred below 20 ppm. The HF content maybe determined by titration according to potentiometric orpotentiographic titration method.

The inventive electrolyte composition (A) furthermore contains at leastone conducting salt (ii). Electrolyte composition (A) functions as amedium that transfers ions participating in the electrochemical reactiontaking place in an electrochemical cell. The conducting salt(s) (ii)present in the electrolyte are usually solvated in the aprotic organicsolvent(s) (i). Preferably the conducting salt (ii) is a lithium salt.The conducting salt is preferably selected from the group consisting of

-   -   Li[F_(6−x)P(C_(y)F_(2y+1))_(x)], wherein x is an integer in the        range from 0 to 6 and y is an integer in the range from 1 to 20;        -   Li[B(R^(I))₄], Li[B(R^(I))₂(OR^(II)O)] and Li[B(OR^(II)O)₂]            wherein each R^(I) is independently from each other selected            from F, Cl, Br, I, C₁-C₄ alkyl, C₂-C₄ alkenyl, and C₂-C₄            alkynyl, wherein alkyl, alkenyl, and alkynyl may be            substituted by one or more OR^(III), wherein R^(III) is            selected from C₁-C₆ alkyl, C₂-C₆ alkenyl, and C₂-C₆ alkynyl,            and        -   (OR^(II)O) is a bivalent group derived from a 1,2- or            1,3-diol, a 1,2- or 1,3-dicarboxylic acid or a 1,2- or            1,3-hydroxycarboxylic acid, wherein the bivalent group forms            a 5- or 6-membered cycle via the both oxygen atoms with the            central B-atom;    -   LiClO₄; LiAsF₆; LiCF₃SO₃; Li₂SiF₆; LiSbF₆; LiAlCl₄,        Li[N(SO₂F)₂], lithium tetrafluoro (oxalato) phosphate; lithium        oxalate; and    -   salts of the general formula Li[Z(CnF_(2n+1)SO₂)_(m)], where m        and n are defined as follows:        -   m=1 when Z is selected from oxygen and sulfur,        -   m=2 when Z is selected from nitrogen and phosphorus,        -   m=3 when Z is selected from carbon and silicon, and        -   n is an integer in the range from 1 to 20.

Suited 1,2- and 1,3-diols from which the bivalent group (OR^(II)O) isderived may be aliphatic or aromatic and may be selected, e.g., from1,2-dihydroxybenzene, propane-1,2-diol, butane-1,2-diol,propane-1,3-diol, butan-1,3-diol, cyclohexyl-trans-1,2-diol andnaphthalene-2,3-diol which are optionally are substituted by one or moreF and/or by at least one straight or branched non fluorinated, partlyfluorinated or fully fluorinated C₁-C₄ alkyl group. An example for such1,2- or 1,3-diole is 1,1,2,2-tetra(trifluoromethyl)-1,2-ethane diol.

“Fully fluorinated C₁-C₄ alkyl group” means, that all H-atoms of thealkyl group are substituted by F.

Suited 1,2- or 1,3-dicarboxylic acids from which the bivalent group(OR^(II)O) is derived may be aliphatic or aromatic, for example oxalicacid, malonic acid (propane-1,3-dicarboxylic acid), phthalic acid orisophthalic acid, preferred is oxalic acid. The 1,2- or 1,3-dicarboxylicacid are optionally substituted by one or more F and/or by at least onestraight or branched non fluorinated, partly fluorinated or fullyfluorinated C₁-C₄ alkyl group.

Suited 1,2- or 1,3-hydroxycarboxylic acids from which the bivalent group(OR^(II)O) is derived may be aliphatic or aromatic, for examplesalicylic acid, tetrahydro salicylic acid, malic acid, and 2-hydroxyacetic acid, which are optionally substituted by one or more F and/or byat least one straight or branched non fluorinated, partly fluorinated orfully fluorinated C₁-C₄ alkyl group. An example for such 1,2- or1,3-hydroxycarboxylic acids is 2,2-bis(trifluoromethyl)-2-hydroxy-aceticacid.

Examples of Li[B(R^(I))₄], Li[B(R^(I))₂(OR^(II)O)] and Li[B(OR^(II)O)₂]are LiBF₄, lithium difluoro oxalato borate and lithium dioxalate borate.

Preferably the at least one conducting salt (ii) is selected from LiPF₆,LiBF₄, and LiPF₃(CF₂CF₃)₃, more preferred the conducting salt (ii) isselected from LiPF₆ and LiBF₄, and the most preferred conducting salt(ii) is LiPF₆.

The at least one conducting salt (ii) is usually present at a minimumconcentration of at least 0.01 wt.-%, preferably of at least 1 wt.-%,and more preferred of at least 5 wt.-%, based on the total weight of theelectrolyte composition. Usually the upper concentration limit for theat least one conducting salt (ii) is 25 wt.-%, based on the total weightof the electrolyte composition.

The inventive electrolyte composition (A) contains as component (iii) atleast one compound of formula (I)

wherein

-   X is either carbon, S(O) or P(OR⁸), preferably X is carbon or S(O);-   n can be 0 or 1, preferably n is 1;-   R¹ is selected from C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆    alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇    (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl,    aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl    may be substituted by one or more substituents selected from F, CN,    C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇    (hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵,    OS(O)₂R⁵, and S(O)₂OR⁵;-   R², R³ and R⁴ are independently from each other selected from H,    C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆    (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃    aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl,    (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl,    and (hetero)aryl may be substituted by one or more substituents    selected from F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆    alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷,    OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR⁷;    -   or R² and R³ are combined with each other and form together with        the C—C single bond of the oxirane cycle a 5 to 7 membered        hydrocarbon cycle which is substituted by at least one group        selected from (O) and oxiranyl, and/or two adjacent C-atoms of        the hydrocarbon carbon cycle form together with an O-atom an        additional oxirane cycle;-   R⁵, R⁶, R⁷ and R⁸ are independently from each other selected from H,    C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆    (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃    aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl,    (hetero)cycloalkenyl, alkynyl, and (hetero)aryl may be substituted    by one or more substituents selected from F, CN, and oxiranyl.

For the sake of clarity, in the case that R² and R³ are combined to formtogether with the C—C single bond of the oxirane cycle a 5 to 7 memberedhydrocarbon cycle R⁴ is defined as above.

The term “C₁-C₆ alkyl” as used herein means a straight or branchedsaturated hydrocarbon group with 1 to 6 carbon atoms having one freevalence and includes, e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl, isobutyl, tert-butyl, n-pentyl, iso-pentyl,2,2-dimethylpropyl, n-hexyl, iso-hexyl, 2-ethyl hexyl, n-heptyl,iso-heptyl, n-octyl, iso-octyl, n-nonyl, n-decyl and the like. Preferredare C₁-C₄ alkyl groups and most preferred are 2-propyl, methyl andethyl.

The term “C₃-C₆ (hetero)cycloalkyl” as used herein means a cyclicsaturated hydrocarbon group with 3 to 6 carbon atoms having one freevalence wherein one or more C-atoms may be replaced by N, O or S.Examples of C₃-C₆ cycloalkyl include cyclopropyl, cyclobutyl,cyclopentyl and cyclohexyl, preferred is cyclohexyl. Examples of C₃-C₆hetero cycloalkyl are oxiranyl and tetrahydrofuryl, preferred isoxiranyl (1,2-epoxy-ethyl). The term “oxirane cycle” as used hereinmeans a substituted or unsubstituted oxirane cycle and includesoxiranyl, 1,1-oxiranediyl, and 1,2-oxiranediyl.

The term “C₂-C₆ alkenyl” as used herein refers to an unsaturatedstraight or branched hydrocarbon group with 2 to 6 carbon atoms havingone free valence. Unsaturated means that the alkenyl group contains atleast one C═C double bond. C₂-C₆ alkenyl includes for example ethenyl,1-propenyl, 2-propenyl, 1-n-butenyl, 2-n-butenyl, iso-butenyl,1-pentenyl, 1-hexenyl and the like. Preferred are C₂-C₄ alkenyl groupsand in particular ethenyl and propenyl (1-propen-3-yl, allyl).

The term “C₃-C₆ (hetero)cycloalkenyl” as used herein refers to a cyclicunsaturated hydrocarbon group with 3 to 6 carbon atoms having one freevalence wherein one or more C-atoms may be replaced by N, O or S.Unsaturated means that the cycloalkenyl group contains at least one C—Cdouble bond. Examples of C₃-C₆ (hetero)cycloalkenyl are cyclopropenyl,cyclobutenyl, cyclopentenyl, and cyclohexenyl.

The term “C₂-C₆ alkynyl” as used herein refers to an unsaturatedstraight or branched hydrocarbon group with 2 to 6 carbon atoms havingone free valence, wherein the hydrocarbon group contains at least oneC—C triple bond. C₂-C₁₀ alkynyl includes for example ethynyl,1-propynyl, 2-propynyl, 1-n-butinyl, 2-n-butynyl, iso-butinyl,1-pentynyl, 1-hexynyl and the like. Preferred is C₂-C₄ alkynyl, inparticular propynyl (1-propyn-3-yl, propargyl).

The term “C₅-C₇ (hetero)aryl” as used herein denotes an aromatic 5- to7-membered hydrocarbon cycle having one free valence, wherein one ormore C-atom may be replaced by N, O or S. An example of C₅-C₇ aryl isphenyl, examples of C₅-C₇ heteroaryl are pyrrolyl, furanyl, thiophenyl,pyridinyl, pyranyl, and thiopyranyl.

The term “C₇-C₁₃ aralkyl” as used herein denotes an aromatic 5- to7-membered hydrocarbon cycle substituted by one or more C₁-C₆ alkyl. TheC₇-C₁₃ aralkyl group contains in total 7 to 13 C-atoms and has one freevalence. The free valence may be located at the aromatic cycle or at aC₁-C₆ alkyl group, i.e. C₇-C₁₃ aralkyl group may be bound via thearomatic part or via the alkyl part of the aralkyl group. Examples ofC₇-C₁₃ aralkyl are methylphenyl, 1,2-dimethylphenyl, 1,3-dimethylphenyl,1,4-dimethylphenyl, ethylphenyl, 2-propylphenyl, and the like.

The term “R² and R³ are combined with each other and form together withthe C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarboncycle” is intended to mean that the 5- to 7-membered hydrocarbon cycleis formed by R² and R³ together with the C—C single bond of the oxiranecycle of general formula (I), i.e. the two C-atoms of the oxirane cycleare 2 members of the 5- to 7-membered hydrocarbon cycle.

According to one embodiment of the present invention the at least onecompound of formula (I) is selected from compounds of formula (I)

wherein

-   R¹ is selected from C₁-C₆ alkyl;-   R² and R³ are independently from each other selected from H and    C₁-C₆ alkyl; and-   R⁴ is selected from H, C₁-C₆ alkyl, and C(O)OR⁷, wherein R⁷ is    preferably C₁-C₆ alkyl.

This embodiment includes compounds of formula (I) wherein R¹ is selectedfrom C₁-C₆ alkyl and R² and R³ are H and R⁴ is H or C(O)OR⁷, wherein R⁷is preferably C₁-C₆ alkyl, and compounds of formula (I) wherein R¹ isselected from C₁-C₆ alkyl and R², R³ and R⁴ are hydrogen.

According to another embodiment of the present invention the at leastone compound of formula (I) is selected from compounds of formula (I)wherein X is a carbon and n equals 1.

Within this embodiment it is preferred if R¹ is selected from C₁-C₆alkyl and R² and R³ are selected from hydrogen and C₁-C₆ alkyl, and R⁴is selected from H, C₁-C₆ alkyl, and C(O)OR⁷, wherein R⁷ is preferablyC₁-C₆ alkyl. This embodiment includes compounds of formula (I) whereinR¹ is selected from C₁-C₆ alkyl, R² and R³ are hydrogen, and R⁴ isC(O)OR⁷, wherein R⁷ is selected from C₁-C₆ alkyl, and compounds offormula (I) wherein R¹ is selected from C₁-C₆ alkyl, and R², R³ and R⁴are hydrogen.

According to another embodiment of the invention electrolyte composition(A) contains compounds of formula (I) wherein X is S(O) and n equals 1.Preferably R¹ is selected from C₁-C₆ alkyl, R² and R³ are hydrogen andR⁴ is H or C(O)OR⁷ wherein R⁷ is selected from C₁-C₆ alkyl.

The compounds of formula (I) described above may contain only oneoxirane cycle or at least two oxirane cycles.

The concentration of the at least one compound of formula (I) in theinventive electrolyte composition (A) usually is 0.001 to 10 wt.-%,preferred 0.01 to 2.5 wt.-%, more preferred 0.01 to 2 wt.-%, and mostpreferred 0.01 to 1.5 wt.-%, based on the total weight of theelectrolyte composition (A).

The electrolyte composition (A) may contain at least one furtheradditive (iv) which is selected from the group consisting of vinylenecarbonate and its derivatives, vinyl ethylene carbonate and itsderivatives, methyl ethylene carbonate and its derivatives, lithium(bisoxalato) borate, lithium difluoro (oxalato) borate, lithiumtetrafluoro (oxalato) phosphate, lithium oxalate, 2-vinyl pyridine,4-vinyl pyridine, cyclic exo-methylene carbonates, sultones, cyclic andacyclic sulfonates, cyclic and acyclic sulfites, cyclic and acyclicsulfinates, organic esters of inorganic acids, acyclic and cyclicalkanes having a boiling point at 1 bar of at least 36° C., and aromaticcompounds, optionally halogenated cyclic and acyclic sulfonylimides,optionally halogenated cyclic and acyclic phosphate esters, optionallyhalogenated cyclic and acyclic phosphines, optionally halogenated cyclicand acyclic phosphites including, optionally halogenated cyclic andacyclic phosphazenes, optionally halogenated cyclic and acyclicsilylamines, optionally halogenated cyclic and acyclic halogenatedesters, optionally halogenated cyclic and acyclic amides, optionallyhalogenated cyclic and acyclic anhydrides, ionic liquids, and optionallyhalogenated organic heterocycles. The additive (iv) is preferablyselected to be different from the compound selected as conducting salt(ii) present in the respective electrolyte composition (A). Preferablyadditive (iv) is also different from the at least one organic aproticsolvent (i) present in the respective electrolyte composition (A).

Preferred ionic liquids according to the present invention are selectedfrom ionic liquids of formula [K]⁺[L]⁻ in which:

[K]⁺ denotes a cation, preferably reduction-stable, selected from thecation groups of the general formulae (II) to (IX)

wherein

-   R denotes H, C₁- to C₆-alkyl, C₂- to C₆-alkenyl, and phenyl,    preferably methyl, ethyl, and propyl;-   R^(A) denotes —(CH₂)_(s)—O—C(O)—R, —(CH₂)_(s)—C(O)—OR,    —(CH₂)_(s)—S(O)₂—OR, —(CH₂)_(s)—O—S(O)₂—R, —(CH₂)_(s)—O—S(O)₂—OR,    —(CH₂)_(s)—O—C(O)—OR, —(CH₂)_(s)—HC═CH—R, —(CH₂)_(s)—CN,

-   -   wherein individual CH₂ groups may be replaced by O, S or NR and        s=1 to 8, preferably s=1 to 3;

-   X^(A) denotes CH₂, O, S or NR^(B);

-   R^(B) denotes H, C₁- to C₆-alkyl, C₂- to C₆-alkenyl, phenyl, and    —(CH₂)_(s)—CN with s=1 to 8, preferably s=1 to 3; preferably R^(B)    is methyl, ethyl, propyl or H;    and

-   [L]⁻ denotes an anion selected from the group BF₄ ⁻, PF₆ ⁻,    [B(C₂O₄)₂]⁻, [F₂B(C₂O₄)]⁻, [N(S(O)₂F)₂]⁻,    [F_(p)P(C_(q)F_(2q+1))_(6−p)]⁻, [N(S(O)₂C_(q)F_(2q+1))₂]⁻,    [(C_(q)F_(2q+1))₂P(O)O]⁻, [C_(q)F_(2q+1)P(O)O₂]²⁻,    [OC(O)C_(q)F_(2q+1)]⁻, [OS(O)₂C_(q)F_(2q+1)]⁻;    [N(C(O)C_(q)F_(2q+1))₂]⁻;    [N(C(O)C_(q)F_(2q+1))(S(O)₂C_(q)F_(2q+1))]⁻;    [N(C(O)C_(q)F_(2q+1))(C(O))F]⁻; [N(S(O)₂C_(q)F_(2q+1))(S(O)₂F)]⁻;    [C(C(O)C_(q)F_(2q+1))₃]⁻, and [C(S(O)₂C_(q)F_(2q+1))₃N(SO₂CF₃)₂]⁻,    wherein p is an integer in the range from 0 to 6 and q is an integer    in the range from 1 to 20, preferably q is an integer in the ranger    from 1 to 4.

Preferred ionic liquids for use as additive (iv) are ionic liquids offormula [K][L] in which [K] is selected from pyrrolidinium cations offormula (II) with X is CH₂ and s is an integer in the range of from 1 to3 and [L] is selected from the group consisting of BF₄ ⁻, PF₆ ⁻,[B(C₂O₄)₂]⁻, [F₂B(C₂O₄)]⁻, [N(S(O)₂F)₂]⁻, [N(SO₂C₂F₅)₂ ²]⁻,[F₃P(C₂F₅)₃]⁻, and [F₃P(C₄F₉)₃]⁻.

If one or more further additives (iv) are present in the electrolytecomposition (A), the total concentration of further additives (iv) is atleast 0.001 wt.-%, preferred 0.005 to 5 wt.-% and most preferred 0.01 to2 wt.-%, based on the total weight of the electrolyte composition (A).

A further object of the present invention is the use of at least onecompound of formula (I) as defined above as additive for electrolytes inelectrochemical cells, preferably in lithium ion secondaryelectrochemical cells.

The compounds of general formula (I) are well-suited as film formingadditives in electrochemical cells. The film may be formed on the anodeand/or on the cathode. Preferably the compounds of general formula (I)are used as film forming additives in lithium ion secondaryelectrochemical cells, in particular as additives forming a film on theanode of lithium ion secondary electrochemical cells.

The compounds of general formula (I) are usually added to theelectrolyte composition to yield a concentration of from is 0.001 to 10wt.-%, preferred 0.01 to 2 wt.-% and most preferred 0.01 to 1.5 wt.-%,based on the total weight of the electrolyte composition (A).

Another object of the present invention is an electrochemical cellcomprising

-   (A) the electrolyte composition as described above,-   (B) at least one cathode comprising at least one cathode active    material, and-   (C) at least one anode comprising at least one anode active    material.

Preferably the electrochemical cell is a secondary lithium ionelectrochemical cell, i.e. secondary lithium ion electrochemical cellcomprising a cathode comprising a cathode active material that canreversibly occlude and release lithium ions and an anode comprising aanode active material that can reversibly occlude and release lithiumions. The terms “secondary lithium ion electrochemical cell” and“(secondary) lithium ion battery” are used interchangeably within thepresent invention.

The at least one cathode active material preferably comprises a materialcapable of occluding and releasing lithium ions selected from lithiatedtransition metal phosphates and lithium ion intercalating transitionmetal oxides.

Examples of lithiated transition metal phosphates are LiFePO₄ andLiCoPO₄, examples of lithium ion intercalating transition metal oxidesare transition metal oxides with layer structure having the generalformula (X) Li_((1+z))[Ni_(a)CO_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0to 0.3; a, b and c may be same or different and are independently 0 to0.8 wherein a+b+c=1; and −0.1≦e≦0.1, and manganese-containing spinels ofgeneral formula (XI) Li_(1+t)M_(2−t)O_(4−d) wherein d is 0 to 0.4, t is0 to 0.4 and M is Mn and at least one further metal selected from thegroup consisting of Co and Ni, andLi_((1+g))[Ni_(h)CO_(i)Al_(j)]_((1−g))O_(2+k). Typical values for g, h,l, j and k are: g=0, h=0.8 to 0.85, i=0.15 to 0.20, j=0.02 to 0.03 andk=0.

In one preferred embodiment the cathode active material is selected fromLiCoPO₄. The cathode containing LiCoPO₄ as cathode active material mayalso be referred to as LiCoPO₄ cathode. The LiCoPO₄ may be doped withFe, Mn, Ni, V, Mg, Al, Zr, Nb, Tl, Ti, K, Na, Ca, Si, Sn, Ge, Ga, B, As,Cr, Sr, or rare earth elements, i.e., a lanthanide, scandium andyttrium. LiCoPO₄ with olivine structure is particularly suited accordingthe present invention due to its high operating voltage (red-oxpotential of 4.8 V vs. Li/Li⁺), flat voltage profile and a hightheoretical capacity of about 170 mAh/g. The cathode may comprise aLiCoPO₄/C composite material. The preparation of a suited cathodecomprising a LiCoPO₄/C composite material is described in Markevich etal., Electrochem. Comm., 2012, 15, 22-25.

In another preferred embodiment of the present invention the cathodeactive material is selected from transition metal oxides with layerstructure having the general formula (X)Li_((1+z))[Ni_(a)Co_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0 to 0.3; a,b and c may be same or different and are independently 0 to 0.8 whereina+b+c=1; and −0.1≦e≦0.1. Preferred are transition metal oxides withlayer structure having the general formula (X)Li_((1+z))[Ni_(a)CO_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0.05 to 0.3,a=0.2 to 0.5, b=0 to 0.3 and c=0.4 to 0.8 wherein a+b+c=1; and−0.1≦e≦0.1. In one embodiment of the present invention, the transitionmetal oxides with layer structure of general formula (X) are selectedfrom those in which [Ni_(a)Co_(b)Mn_(c)] is selected fromNi_(0.33)Co₀Mn_(0.66), Ni_(0.25)Co₀Mn_(0.75),Ni_(0.35)Co_(0.15)Mn_(0.5), Ni_(0.21)Co_(0.08)Mn_(0.71) andNi_(0.22)Co_(0.12)Mn_(0.66), in particular preferred areNi_(0.21)Co_(0.08)Mn_(0.71) and Ni_(0.22)Co_(0.12)Mn_(0.66). Thetransition metal oxides of general formula (X) are also called HighEnergy NCM (HE-NCM) since they have higher energy densities than usualNCMs. Both HE-NCM and NCM have operating voltage of about 3.3 to 3.8 Vagainst Li/Li⁺, but high cut off voltages (>4.6 V) have to be used forcharging HE-NCMS to actually accomplish full charging and to benefitfrom their higher energy density.

According to a further preferred embodiment of the present invention thecathode active material is selected from manganese-containing spinels ofgeneral formula (XI) Li_(1+t)M_(2−t)O_(4−d) wherein d is 0 to 0.4, t is0 to 0.4 and M is Mn and at least one further metal selected from thegroup consisting of Co and Ni. An example of a suitedmanganese-containing spinel of general formula (XI) isLiNi_(0.5)Mn_(1.5)O_(4−d). These spinels are also called HE (highenergy)-spinels.

Many elements are ubiquitous. For example, sodium, potassium andchloride are detectable in certain very small proportions in virtuallyall inorganic materials. In the context of the present invention,proportions of less than 0.5% by weight of cations or anions aredisregarded, i.e. amounts of cations or anions below 0.5% by weight areregarded as non-significant. Any lithium ion-containing transition metaloxide comprising less than 0.5% by weight of sodium is thus consideredto be sodium-free in the context of the present invention.Correspondingly, any lithium ion-containing mixed transition metal oxidecomprising less than 0.5% by weight of sulfate ions is considered to besulfate-free in the context of the present invention.

The cathode may further comprise electrically conductive materials likeelectrically conductive carbon and usual components like binders.Compounds suited as electrically conductive materials and binders areknown to the person skilled in the art. For example, the cathode maycomprise carbon in a conductive polymorph, for example selected fromgraphite, carbon black, carbon nanotubes, graphene or mixtures of atleast two of the aforementioned substances. In addition, the cathode maycomprise one or more binders, for example one or more organic polymerslike polyethylene, polyacrylonitrile, polybutadiene, polypropylene,polystyrene, polyacrylates, polyvinyl alcohol, polyisoprene andcopolymers of at least two comonomers selected from ethylene, propylene,styrene, (meth)acrylonitrile and 1,3-butadiene, especiallystyrene-butadiene copolymers, and halogenated (co)polymers likepolyvinylidene chloride, polyvinyl chloride, polyvinyl fluoride,polyvinylidene fluoride (PVdF), polytetrafluoroethylene, copolymers oftetrafluoroethylene and hexafluoropropylene, copolymers oftetrafluoroethylene and vinylidene fluoride and polyacrylnitrile.

Furthermore, the cathode may comprise a current collector which may be ametal wire, a metal grid, a metal web, a metal sheet, a metal foil or ametal plate. A suited metal foil is aluminum foil.

According to one embodiment of the present invention the cathode has athickness of from 25 to 200 μm, preferably of from 30 to 100 μm, basedon the whole thickness of the cathode without the thickness of thecurrent collector.

The anode (C) comprised within the lithium ion secondary battery of thepresent invention comprises an anode active material that can reversiblyocclude and release lithium ions. In particular carbonaceous materialthat can reversibly occlude and release lithium ions can be used asanode active material. Carbonaceous materials suited are crystallinecarbon such as a graphite material, more particularly, natural graphite,graphitized cokes, graphitized MCMB, and graphitized MPCF; amorphouscarbon such as coke, mesocarbon microbeads (MCMB) fired below 1500° C.,and mesophase pitch-based carbon fiber (MPCF); hard carbon and carbonicanode active material (thermally decomposed carbon, coke, graphite) suchas a carbon composite, combusted organic polymer, and carbon fiber.

Further anode active materials are lithium metal, or materialscontaining an element capable of forming an alloy with lithium.Non-limiting examples of materials containing an element capable offorming an alloy with lithium include a metal, a semimetal, or an alloythereof. It should be understood that the term “alloy” as used hereinrefers to both alloys of two or more metals as well as alloys of one ormore metals together with one or more semimetals. If an alloy hasmetallic properties as a whole, the alloy may contain a nonmetalelement. In the texture of the alloy, a solid solution, a eutectic(eutectic mixture), an intermetallic compound or two or more thereofcoexist. Examples of such metal or semimetal elements include, withoutbeing limited to, titanium (Ti), tin (Sn), lead (Pb), aluminum, indium(In), zinc (Zn), antimony (Sb), bismuth (Bi), gallium (Ga), germanium(Ge), arsenic (As), silver (Ag), hafnium (Hf), zirconium (Zr) yttrium(Y), and silicon (Si). Metal and semimetal elements of Group 4 or 14 inthe long-form periodic table of the elements are preferable, andespecially preferable are titanium, silicon and tin, in particularsilicon. Examples of tin alloys include ones having, as a secondconstituent element other than tin, one or more elements selected fromthe group consisting of silicon, magnesium (Mg), nickel, copper, iron,cobalt, manganese, zinc, indium, silver, titanium (Ti), germanium,bismuth, antimony and chromium (Cr). Examples of silicon alloys includeones having, as a second constituent element other than silicon, one ormore elements selected from the group consisting of tin, magnesium,nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium,germanium, bismuth, antimony and chromium.

A further possible anode active material is silicon which is able tointercalate lithium ions. The silicon may be used in different forms,e.g. in the form of nanowires, nanotubes, nanoparticles, films,nanoporous silicon or silicon nanotubes. The silicon may be deposited ona current collector. The current collector may be a metal wire, a metalgrid, a metal web, a metal sheet, a metal foil or a metal plate.Preferred the current collector is a metal foil, e.g. a copper foil.Thin films of silicon may be deposited on metal foils by any techniqueknown to the person skilled in the art, e.g. by sputtering techniques.One possibility of preparing Si thin film electrodes are described in R.Elazari et al.; Electrochem. Comm. (2012), 14, 21-24. It is alsopossible to use a silicon/carbon composite as anode active materialaccording to the present invention.

Other possible anode active materials are lithium ion intercalatingoxides of Ti.

Preferably the anode active material present in the inventive lithiumion secondary battery is selected from carbonaceous material that canreversibly occlude and release lithium ions, particularly preferred thecarbonaceous material that can reversibly occlude and release lithiumions is selected from crystalline carbon, hard carbon and amorphouscarbon, in particular preferred is graphite. In another preferredembodiment the anode active material present in the inventive lithiumion secondary battery is selected from silicon that can reversiblyocclude and release lithium ions, preferably the anode comprises a thinfilm of silicon or a silicon/carbon composite. In a further preferredembodiment the anode active material present in the inventive lithiumion secondary battery is selected from lithium ion intercalating oxidesof Ti.

The anode and cathode may be made by preparing an electrode slurrycomposition by dispersing the electrode active material, a binder,optionally a conductive material and a thickener, if desired, in asolvent and coating the slurry composition onto a current collector. Thecurrent collector may be a metal wire, a metal grid, a metal web, ametal sheet, a metal foil or a metal plate. Preferred the currentcollector is a metal foil, e.g. a copper foil or aluminum foil.

The inventive lithium ion batteries may contain further constituentscustomary per se, for example separators, housings, cable connectionsetc. The housing may be of any shape, for example cuboidal or in theshape of a cylinder, the shape of a prism or the housing used is ametal-plastic composite film processed as a pouch. Suited separators arefor example glass fiber separators and polymer-based separators likepolyolefin separators.

Several inventive lithium ion batteries may be combined with oneanother, for example in series connection or in parallel connection.Series connection is preferred. The present invention further providesfor the use of inventive lithium ion batteries as described above indevices, especially in mobile devices. Examples of mobile devices arevehicles, for example automobiles, bicycles, aircraft, or water vehiclessuch as boats or ships. Other examples of mobile devices are those whichare portable, for example computers, especially laptops, telephones orelectrical power tools, for example from the construction sector,especially drills, battery-driven screwdrivers or battery-drivenstaplers. But the inventive lithium ion batteries can also be used forstationary energy stores.

The invention is illustrated by the examples which follow, which do not,however, restrict the invention.

1. Preparation of Compounds Compound 1.

Racemic methyl glycidate 1 was prepared according to the followingliterature procedure: Facial Selectivity and Stereospecificity in the(4+3) Cycloaddition of Epoxy Enol Silanes. Lo, Brian; Chiu, Pauline,Organic Letters 2011, 13(5), pages 864 to 867.

Compound 2.

Methyl 2,3-epoxy-2-methylacrylate 2 was prepared according to thefollowing literature procedure: Efficient Epoxidation ofElectron-Deficient Olefins with a Cationic Manganese Complex. Murphy,Andrew; Dubois, Geraud; Stack, T. D. P., Journal of the AmericanChemical Society 2003, 125(18), pages 5250 to 5251.

Compound 3.

Dimethyl 2,3-oxiranedicarboxylate 3 was prepared according to thefollowing literature procedure: Preparation of dicarboxylate analoguesof cerulenin. Moseley, Jonathan D.; Staunton, James, Journal ofHeterocyclic Chemistry 2005, 42(5), pages 819 to 830.

Compound 4.

4,8-Dioxatricyclo[5.1.0.03,5]octane-2,6-dione 4 was prepared accordingto the following literature procedure: Acetal and ketal deprotectionusing montmorillonite K10: the first synthesis ofsyn-4,8-dioxatricyclo[5.1.0.03,5]-2,6-octanedione. Gautier, Elisabeth C.L.; Graham, Andrew E.; McKillop, Alexander; Standen, Stephen P.; Taylor,Richard J. K., Tetrahedron Letters 1997, 38(11), pages 1881 to 1884.

Compound 5.

Dimethyl-α,β-epoxyethylphosphonate 5 was prepared according to thefollowing literature procedure: Preparation and ring opening reactionsof α,β- and α,γ-epoxyalkylphosphonates. The proton magnetic resonancespectra of vicinally substituted ethyl- and propyl-phosphonates.Griffin, Claibourne E.; Kundu, S. K., Journal of Organic Chemistry 1969,34(6), pages 1532 to 1539.

Compounds 1 to 5 are summarized in Table 1.

TABLE 1 Formulae of some compounds according to general formula (I)Compound Name Structure 1 Methyl glycidate

2 Methyl 2,3-epoxy- 2-methylacrylate

3 Dimethyl 2,3- oxiranedicarboxylate

4 4,8-Dioxatricyclo [5.1.0.03,5] octane-2,6-dione

5 Dimethyl--epoxyethyl- phosphonate

2. Electrochemical Cells 2.1 Half Cells

The additives were investigated towards film formation in coin cells(half cells). Graphite-coated tapes (2.2 mAh/g) and metal lithium wereused as electrodes. A glass-fiber filter separator (Whatmann GF/D) wasused as the separator, which was soaked with 120 μl electrolyte. Cointype half cells (type 2032—Hohsen) were prepared with LiPF₆, which wasdissolved in a 3:7 (by weight) mixture of ethylenecarbonate/ethyl-methyl carbonate yielding a 1 M LiPF₆ solution (baseelectrolyte composition). 2 wt.-% of additive was added to theelectrolyte mixture. All cells were assembled in an argon-filled glovebox (Unilab, MBraun) having oxygen and water levels below 10 ppm.Afterwards the test cells were transferred to a battery test station.Electrochemical cycling was done using a Maccor battery test system. Thehalf cells were initially held at open circuit potential for 6 hours andthe graphite anodes were subsequently lithiated until the cell voltagereached 0.025 V. Afterwards the graphite anodes were delithiated untilthe cell voltage had a value of 1.5 V. This test was performed at 0.01 Crate. Measurements were carried out at room temperature (25° C.).

The reduction potentials obtained from differential capacity vs. voltageplots of coin cells (Li/graphite) containing the base electrolytecomposition with 2 wt.-% electrolyte additive are summarized in Table 2.

2.2 Full Cells

Corresponding electrolyte additives were further investigated in fullcells at ambient temperature (25° C.). The coin type cell contains astainless steel spacer to contact the graphite anode and a stainlesssteel casing bottom to contact the backside of the cathode. The cathodematerial NCM 111 (LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂) was used to makecathode tapes with a capacity of 2 mAh/cm². Graphite-coated tapes(capacity about 2.2 mAh/cm²) were used as anodes. A glass-fiber filterseparator (Whatmann GF/D) was used as the separator, which was soakedwith 120 μl electrolyte. Coin type full cells were prepared with LiPF₆(Kanto Denka Koyo Co. Ltd), which was dissolved in a 3:7 (by weight)mixture of ethylene carbonate/ethyl methylcarbonate yielding a 1 M LiPF₆solution (comparative sample). 0.5 wt.-% of a compound of formula (I)were added to the electrolyte mixture. All cells were assembled in anargon-filled glove box (Unilab, MBraun) having oxygen and water levelsbelow 10 ppm. Afterwards the test cells were transferred to a batterytest station. Electrochemical cycling (charging/discharging) was doneusing a Maccor battery test system. The full cells were initially heldat open circuit potential for 6 hours and subsequently charged to 4.2 V.Afterwards the cells were discharged to a low voltage cutoff of 3.0 V.The first two cycles were performed at 0.1 C rate followed by cycling at0.5 C (10 cycles), rate test up to 10 C and continuous cycling at 1 Cbetween 3.0 and 4.2 V. All measurements were carried out in climatechambers. The internal resistance was determined by DC (discharge) pulsemethod (10 sec at 0.1 C, 10 sec at 1 C). The cell capacities obtainedafter 100 cycles and internal resistance values after 100 cycles of theNCM 111/graphite full cells are summarized in Table 2. The initialcapacity of the cells was about 3.3 mAh.

TABLE 2 Test results Internal resist- Capacity ance Reduction afterafter potential 100 100 (V vs Li/Li⁺) cycles cycles Compound of formula(I) [V] [mAh] [Ω cm²]

1.15 3.28 33.66

1.35 3.26 33.20 None (comparative) 0.78 3.26 35.8 

The inventive electrolyte compositions containing compounds of formula(I) show lower or only slightly increased internal resistances incomparison to a similar electrolyte composition containing no compoundof formula (I) and have a clearly higher reduction potential indicatingthe film formation on the anode. The reduction peak observed for thebase electrolyte composition containing no additive is significantlyreduced for the inventive electrolyte compositions.

1: An electrolyte composition (A), comprising (i) an aprotic organicsolvent; (ii) a conducting salt; (iii) a compound of formula (I)

wherein X is either carbon, S(O) or P(OR⁸) n is 0 or 1; R¹ is selectedfrom the group consisting of C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl,C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇(hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl,aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryloptionally substituted by one or more substituents selected from thegroup consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆alkenyl, C₅-C₇ (hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵,OC(O)C(O)OR⁵, OS(O)₂R⁵, and S(O)₂OR⁵; R², R³ and R⁴ are independentlyselected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆(hetero)cycloalkyl, C₂-C₆ alkenylC₃-C₆ (hetero)cycloalkenyl, C₂-C₆alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, andS(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl,(hetero)cycloalkenyl, alkynyl, and (hetero)aryl optionally substitutedby one or more substituents selected from the group consisting of F, CN,C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇(hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷,OS(O)₂R⁷, and S(O)₂OR⁷; or R² and R³ are combined with each other andform together with a C—C single bond of the oxirane cycle a 5 to 7membered hydrocarbon cycle which is substituted by at least one selectedfrom the group consisting of O and oxiranyl, and/or two adjacent C-atomsof the hydrocarbon carbon cycle form together with an O-atom anadditional oxirane cycle; R⁵, R⁶, R⁷ and R⁸ are independently selectedfrom the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl,C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇(hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl,aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)arylomionally substituted by one or more substituents selected from thegroup consisting of F, CN, and oxiranyl; and (iv) optionally anadditive. 2: The electrolyte composition (A) according to claim 1,wherein R¹ is C₁-C₆ alkyl; R² and R³ are independently H or C₁-C₆ alkyl;and R⁴ is H, C₁-C₆ alkyl, or C(O)OR⁷. 3: The electrolyte composition (A)according to claim 1, wherein R¹ is C₁-C₆ alkyl; and R², R³ and R⁴ arehydrogen. 4: The electrolyte composition (A) according to claim 1,wherein X is a carbon; and n is
 1. 5: The electrolyte composition (A)according to claim 1, wherein X is carbon; n is 1; R¹ is C₁-C₆ alkyl;and R², R³ and R⁴ are hydrogen. 6: The electrolyte composition (A)according to claim 1, wherein X is carbon; n is 1; R² is C₁-C₆ alkyl; R²and R³ are hydrogen; R⁴ is C(O)OR⁷; and R⁷ is C₁-C₆ alkyl. 7: Theelectrolyte composition (A) according to claim 1, wherein X is S(O); andn is
 1. 8: The electrolyte composition (A) according to claim 1, whereinthe compound of formula (I) comprises at least two oxirane cycles. 9:The electrolyte composition (A) according to claim 1, wherein theaprotic organic solvent (i) is selected from (a) a cyclic or noncyclicorganic carbonate, which is optionally partly halogenated, (b) adi-C₁-C₁₀-alkylether, which is optionally partly halogenated, (c) adi-C₁-C₄-alkyl-C₂-C₆-alkylene ether or polyether, which is optionallypartly halogenated, (d) a cyclic ether, which is optionally partlyhalogenated, (e) a cyclic or acyclic acetal or ketal, which isoptionally partly halogenated, (f) an orthocarboxylic acid ester, whichis optionally partly halogenated, (g) a cyclic or noncyclic ester of acarboxylic acid, which is optionally partly halogenated, (h) a cyclic ornoncyclic sulfone, which is optionally partly halogenated, (i) a cyclicor noncyclic nitrile or dinitrile, which is optionally partlyhalogenated, and (j) an ionic liquid, which is optionally partlyhalogenated. 10: The electrolyte composition (A) according to claim 1,wherein the conducting salt (ii) is selected from the group consistingof Li[F_(6−x)P(C_(y)F_(2y+1))_(x))], wherein x is an integer of from 0to 6 and y is an integer of from 1 to 20; Li[B(R⁹)₄], Li[B(R⁹)₂(OR¹⁰O)]and Li[B(OR¹⁰O)₂] wherein each R⁹ is independently selected from thegroup consisting of F, Cl, Br, I, C₁-C₄ alkyl, C₂-C₄ alkenyl, and C₂-C₄alkynyl, wherein alkyl, alkenyl, and alkynyl optionally substituted byone or more OR¹¹, wherein R¹¹ is selected from the group consisting ofC₁-C₆ alkyl, C₂-C₆ alkenyl, and C₂-C₆ alkynyl, and (OR¹⁰O) is a bivalentgroup derived from a 1,2- or 1,3-diol, a 1,2- or 1,3-dicarboxlic acid ora 12- or 1,3-hydroxycarboxylic acid, wherein the bivalent group forms a5- or 6-membered cycle via both oxygen atoms with a central B-atom;LiClO₄; LiAsF₆; LiCF₃SO₃; Li₂SiF₆; LiSbF₆; LiAlCl₄, Li[N(SO₂F)₂],lithium tetrafluoro (oxalato) phosphate; lithium oxalate; and a salt offormula Li[Z(C_(n)F_(2n+1)SO₂)_(m)], where m and n are defined asfollows: m=1 when Z is oxygen or sulfur, m=2 when Z is from nitrogen orphosphorus, m=3 when Z is carbon or silicon, and n is an integer of from1 to
 20. 11: The electrolyte composition (A) according to claim 1,comprising the additive (iv) which is selected from the group consistingof vinylene carbonate and a derivative thereof, vinyl ethylene carbonateand a derivative thereof, methyl ethylene carbonate and a derivativethereof, lithium (bisoxalato) borate, lithium difluoro (oxalato) borate,lithium tetrafluoro (oxalato) phosphate, lithium oxalate, 2-vinylpyridine, 4-vinyl pyridine, A cyclic exo-methylene carbonate, a sultone,an organic ester of an inorganic acid, a acyclic or a cyclic alkanehaving a boiling point at 1 bar of at least 36° C., and an aromaticcompound, an optionally halogenated cyclic or acyclic sulfonylimide, anoptionally halogenated cyclic or acyclic phosphate ester, optionallyhalogenated cyclic or acyclic phoshine, an optionally halogenated cyclicor acyclic phoshite, an optionally halogenated cyclic or acyclicphosphazene, an optionally halogenated cyclic or acyclic silylamine, anoptionally halogenated cyclic or acyclic halogenated ester, anoptionally halogenated cyclic or acyclic amide, an optionallyhalogenated cyclic or acyclic anhydride an ionic liquid and anoptionally halogenated organic heterocycle. 12: The electrolytecomposition (A) according to claim 1, wherein a concentration of thecompound of formula (I) is 0.001 to 10 wt.-% based on a total weight ofthe electrolyte composition (A). 13: A method for preparing anelectrolyte for an electrochemical cell, the method comprising:introducing a compound of formula (I)

into the electrolyte as an additive, wherein X is carbon, S(O) orP(OR⁸); n is 0 or 1; R¹ is selected from the group consisting of C₁-C₆alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆(hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl,(hetero)cycloalkenyl, alkynyl, and (hetero)aryl optionally substitutedby one or more substituents selected from the group consisting of F, CN,C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇(hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵,OS(O)₂R⁵, and S(O)₂OR⁵; R², R³ and R⁴ are independently selected fromthe group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl,C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl,(hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and(hetero)aryl optionally substituted by one or more substituents selectedfrom the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆(hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷,C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR; or R²and R³ are combined with each other and form together with a C—C singlebond of an oxirane cycle a 5 to 7 membered hydrocarbon cycle which issubstituted by at least one group selected from the group consisting ofO and oxiranyl, and/or two adjacent C-atoms of the hydrocarbon carboncycle form together with an O-atom an additional oxirane cycle; R⁵, R⁶,R⁷ and R⁸ are independently selected from the group consisting of H,C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆(hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl,(hetero)cycloalkenyl, alkynyl, and (hetero)aryl optionally substitutedby one or more substituents selected from the group consisting of F, CN,and oxiranyl. 14: An electrochemical cell, comprising (A) theelectrolyte composition according to claim 1, (B) a cathode comprising acathode active material, and (C) an anode comprising an anode activematerial. 15: The electrochemical cell according to claim 14, which is asecondary lithium ion battery.